Systems for transmitting digital data make use of multicarrier modulation schemes in order to achieve high levels of effectiveness, robustness, and flexibility in operation.
One approach that is effective for implementing the principle of multicarrier modulation is based on filter banks and it is known as the filter bank based multicarrier (FBMC) approach.
The channel for transmitting digital data is subdivided into subchannels, and in each subchannel, the data modulates a subcarrier.
A conventional implementation of filter banks consists in combining a block that is suitable for outputting the discrete Fourier transform of said signal as applied to its input, referred to as an FFT block, and of a size, i.e. having a number of outputs, that is equal to the total number of subchannels in the modulation system, with a polyphase network (PPN) that is a set of digital filters, the number of filters being equal to the size of the discrete Fourier transform known as the fast Fourier transform (FFT) by reference to the fast algorithms used for implementing it.
This technique, principle, calculation, and implementation is described in the work by M. Bellanger, “Digital processing of signals”, Wiley, 2000, at pages 304 to 306 and 309 to 333.
Its application to digital transmission is described in the article by P. Siohan et al., “Analysis and design of OFDM/OQAM systems based on filter bank theory”, IEEE Transactions on Signal Processing, Vol. 50, No. 5, 2002.
The technique in most widespread use for multicarrier modulation is the orthogonal frequency division multiplexing (OFDM) technique, which is likewise based on the FFT. This technique is described in the work by M. Bellanger, “Digital processing of signals”, published by Wiley in 2000, at pages 414 to 418.
In this technique, the FFT is performed at the symbol rate of the multicarrier modulation, where a symbol is constituted by a set of output signals delivered simultaneously by an iFFT block, that is adapted to deliver at its output a signal that corresponds to the inverse Fourier transform of the signal applied to its input.
At the receiver, the sets of signals transmitted by the iFFT block of the transmitter are thus processed by the FFT block in disjoint manner. In the presence of a transmission channel, a guard time is introduced between the symbols on transmission in order to avoid interference between two symbols, and a single-coefficient equalizer compensates the distortions of the channel at multicarrier level, on reception.
The principle of OFDM is simple and well understood, since the FFT forms part of the basic knowledge of engineers in the field of communications. Furthermore, the transmission chain introduces a minimum amount of delay, and this is a characteristic that is well appreciated in numerous applications.
However, the guard time reduces the rate at which data is transmitted, and the filtering properties of the FFT are not sufficient to take full advantage of new concepts in radio communications, in particular cognitive radio.
In contrast, the FBMC technique, which does not require a guard time and which provides high spectral resolution and subchannel independence, lends itself well to cognitive radio.
On reception of the signals carrying the data, the distortion of the transmission channel is compensated by an equalizer in the time domain for each of the subchannels, as explained by T. Ihalainen et al. in the article “Channel equalization in filter bank based multicarrier modulation for wireless communications”, Eurasip Journal on Advances in Signal Processing, Vol. 2007, ID 49389.
When searching for high-performance equalization, the equalizer in each subchannel possesses a plurality of coefficients and associated memories, and it introduces an additional delay in transmission, and that can be a drawback in certain applications.
The FMBC technique is thus more complex to understand and to implement than OFDM, and it gives rise to additional delay in equalization.